Report stops running when published to report server

Hi, I have a report stored locally that has two sub reports within it. The user provides credentials for Server, Database, Username and Password. There parameters get passed to the two sub reports also. When I run the main report from within Visual Studio on my desktop all is fine and all three reports work as expected.

The connection expression is.....
="Data Source=" + Parameters!Server.Value + ";Initial Catalog=" + Parameters!Database.Value + "; user=" + Parameters!Username.Value + ";password=" + Parameters!Password.Value + ";"

When I publish the report to the report server and try to access it through out application I get the error:

'System error attempting to run this report. ErrorCode: rsInvalidDataSourceCredentialSetting
Error Message: The current action cannot be completed. The user data source credentials do not meet the requirements to run this report or shared dataset. Either the user data source credentials are not stored in the report server database, or the user data source is configured not to require credentials but the unattended execution account is not specified.'

Why is it that report runs ok locally (it does not use windows credentials) but not on the report server?

Thanks,
Dean